1. Field of the Invention
The present invention relates to methods of increasing overlay accuracy and overlay displacement measuring devices, and more particularly to a method of increasing overlay accuracy and overlay displacement measuring device for determining a focus plane providing an optimum measuring accuracy.
2. Description of the Background Art
Recently, semiconductor devices such as ICs (Integrated Circuits) and LSIs (Large Scale Integrations) have been increasingly reduced in size. Especially, exposure devices for transferring a circuit pattern on a mask or reticle onto a circuit pattern formed on a semiconductor wafer are required to achieve increasingly high accuracy. As the degree of integration of the devices has been increased, accuracy from 0.20 xcexcm to 0.10 xcexcm is required, and most recently accuracy of 0.10 xcexcm or smaller is required.
Referring to FIGS. 10 to 12B, a method of measuring overlay displacement in a conventional process of manufacturing a semiconductor device will be described.
Referring to FIG. 10, in the conventional method of measuring overlay displacement, positions of overlay inspection marks are measured at measuring points on selected ones of a plurality of chips 102 on a wafer 101. At each of the measuring points, first and second overlay inspection marks 201 and 202 are formed during patterning of an interconnection or the like as shown in FIGS. 11A and 11B. First overlay inspection mark 201 is formed on a substrate, and second overlay inspection mark 202 is formed thereon. A set 204 of first and second overlay inspection marks 201 and 202 is referred to as a Box-in-Box mark 204.
Generally, image recognition is used for measuring a magnitude of displacement of first and second overlay inspection marks 201 and 202 (a magnitude of displacement for Box-in-Box mark 204). A broadband light such as a xenon lamp is used as a light source. By detecting an intensity of the light reflected from positions near edges of first and second overlay inspection marks 201 and 202 by a camera, edge positions of first and second overlay inspection marks 201 and 202 are recognized. Distances a and b shown in FIGS. 11A are obtained, and the magnitude of displacement of first and second overlay inspection marks 201 and 202 are calculated with the following expression (1).
Magnitude of displacement=(axe2x88x92b)/2xe2x80x83xe2x80x83(1)
Around 1991, a TIS (Tool Induced Shift) value of the magnitude of displacement would be used to increase overlay accuracy. The TIS value of displacement represents a difference between measured values of displacement for erecting and inverted images of a wafer, respectively shown in FIGS. 12A and 12B. The TIS value can be calculated with the following expression (2).
TIS value=(a1xe2x88x92b1)/2xe2x88x92(a2xe2x88x92b2)/2xe2x80x83xe2x80x83(2)
a1, b1: distances a and b when erecting
a2, b2: distances a and b when inverted
However, when the TIS value is determined with the inspection mark which has been formed by a prescribed process, the resulting TIS value would be large. Such large TIS value has been resulted from a displaced measurement focus plane.
Then, a method of determining an optimum measurement focus plane using the TIS value had been developed by around 1993. Until now, this method has been used for determining the optimum measurement focus plane.
Now, the method of determining the measurement focus plane will be described.
A wafer 101 is loaded on a stage of an overlay displacement measuring device (not shown) for measuring a magnitude of overlay displacement. A rotational correction of wafer 101 is performed using an arrangement of a plurality of chips 102 formed on wafer 101 as a reference, and a reference point on wafer 101 is selected.
After an output level of a light source and a focus plane of a camera are set, an image of wafer 101 is incorporated.
Then, a magnitude of displacement of first and second overlay inspection marks 201 and 202 are measured in accordance with the above described method of measuring the magnitude of displacement for each of chips 102, so that the TIS value of the magnitude of displacement is obtained.
The TIS value of displacement is repeatedly determined by changing the focus plane and, for each focus plane, a 3S (3 Sigma) of the magnitude of displacement is obtained. Thereafter, the focus plane providing the minimum 3S of the TIS value is selected as the focus plane providing the highest measurement accuracy for the magnitude of displacement.
However, our examination of the above described method of determining the focus plane by 3S of the TIS value has suggested that there is not any relation between 3S of the TIS value and 3S of the measured value of displacement, as shown in FIG. 13. Therefore, even if the focus plane providing the smallest 3S of the TIS value is determined as the measurement focus plane, the magnitude of displacement is not always measured in the optimum manner.
The present invention is made to solve the aforementioned problem. An object of the present invention is to provide an overlay displacement measuring device capable of increasing accuracy for measuring a magnitude of displacement.
Another object of the present invention is to provide an overlay displacement measuring device for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement.
Still another object of the present invention is to provide an overlay displacement measuring device for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement in which an inspection mark having a large magnitude of displacement does not affect a variation in measured values.
Still another object of the present invention is to provide a method of increasing overlay accuracy capable of increasing accuracy for measuring a magnitude of displacement.
Still another object of the present invention is to provide a method of increasing overlay accuracy for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement.
Still another object of the present invention is to provide a method of increasing overlay accuracy for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement in which an inspection mark having a large magnitude of displacement does not affect a variation in measured values.
A method of increasing overlay accuracy according to one aspect of the present invention is performed in an exposure step in a process of manufacturing a semiconductor device. The above mentioned method of increasing overlay accuracy includes: a step of picking up an image of a semiconductor wafer including a plurality of chips each having first and second overlay inspection marks thereon by shifting a focus plane by a predetermined first distance in a predetermined first range with respect to a reference focus plane; a step of calculating a variation in measured values of displacement of the first and second overlay inspection marks for each focus plane; and a step of determining a measurement focus plane based on the variations in the magnitudes of displacement.
The measurement focus plane is determined by the variations in the measured values of displacement of the first and second overlay inspection marks. Unlike the conventional method using the TIS values, the focus plane is determined by using data which is directly related to the measurement of displacement. Therefore, the focus plane enabling accurate measurement of displacement of the first and second overlay inspection marks can be determined.
Preferably, the step of calculating the variation in the measured values of displacement includes: a step of calculating an average value of the measured values of displacement of the first and second overlay inspection marks for each pair of the first and second overlay inspection marks; a step of obtaining a value by subtracting the average value from the measured value for each measured value of each focus plane; and a step of calculating a variation in the values obtained by subtracting the average values from the measured values for each focus plane.
The measurement focus plane can be determined based on variations in values obtained by subtracting the average value of the measured values of displacement from the measured values of displacement. Thus, the overlay inspection mark having a large magnitude of displacement and that having a small magnitude of displacement can be equally used. As a result, accuracy for measuring the magnitude of displacement can be further increased while preventing the inspection mark having the large magnitude of displacement from affecting the variation in the measured values.
An overlay displacement measuring device according to another aspect of the present invention includes: a camera picking up an image of a semiconductor wafer including a plurality of chips each having first and second overlay inspection marks thereon; a displacement measuring portion connected to the camera for measuring a magnitude of displacement of the first and second inspection marks of each chip for each focus plane while controlling a focus of the camera; a measured value variation calculating portion connected to the displacement measuring portion for calculating a variation in measured values of displacement in each focus plane; and a measurement focus plane identifying portion connected to the measured value variation calculating portion for identifying a measurement focus plane by the variations in the measured values.
The measurement focus plane is determined by the variations in the measured values of displacement of the first and second overlay inspection marks. Unlike the conventional method using the TIS values, a focus plane is determined by using data which is directly related to the measurement of displacement. Therefore, the focus plane enabling accurate measurement of displacement of the first and second overlay inspection marks can be identified.
Preferably, the measured value variation calculating portion includes: an average value calculating portion connected to the displacement measuring portion for calculating an average value of the measured values of displacement for each pair of the first and second overlay inspection marks; a subtraction value calculating portion connected to the displacement calculating portion and the average value calculating portion for obtaining a value by subtracting the average value from the measured value for each measured value of each focus plane; and a subtraction value variation calculating portion for calculating a variation in the values obtained by subtracting average values from the measured values for each focus plane.
The measurement focus plane can be determined by the variations in the values obtained by subtracting the average value of the measured values of displacement from the measured value of displacement. Thus, the overlay inspection mark having a large magnitude of displacement and that having a small magnitude of displacement can be equally used. Therefore, accuracy for measuring the magnitude of displacement is further increased while preventing the inspection mark having the large magnitude of displacement from affecting the variation in the measured values.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.